Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
1999-07-22
2001-05-29
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S042000, C180S006200, C180S204000, C180S446000
Reexamination Certificate
active
06240350
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a control apparatus for an electric power steering system in which a steering system of an automobile or a vehicle is provided with a steering assist force generated by a motor. More particularly, the invention relates to a control apparatus for an electric power steering system in which convergence of a yaw rate of a vehicle is ensured.
2. Description of the Related Art
An electric power steering system for assisting a steering system of an automobile or a vehicle by a motor torque operates in such a manner that a steering shaft or a rack shaft is assisted by a driving force of the motor through a speed reducer using a transfer mechanism, such as a gear or a belt. The known electric power steering system performs feedback control of motor current to accurately generate an assist torque (steering assist torque). The feedback control is used for adjusting voltage impressed on the motor so that the difference between a current control value and a motor current detected value decreases. In general, the adjustment of the voltage impressed on the motor is performed by adjusting a duty ratio of pulse width modulation (“PWM”) control. Referring now to
FIG. 1
, a general structure of an electric power steering system is shown. A shaft
2
of a steering wheel
1
is connected to a tie rod
6
of a vehicle wheel through a speed reduction gear
3
, universal joints
4
a and
4
b, and a pinion rack mechanism
5
. A torque sensor
10
for detecting a steering torque of the steering wheel
1
is provided on the shaft
2
. A motor
20
for assisting a steering effort of the steering wheel
1
is connected through a clutch
21
and the speed reduction gear
3
to the shaft
2
. A battery
14
supplies electric power through an ignition key
11
and a relay
13
to a control unit
30
that controls the power steering system. The control unit
30
computes a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor
10
and a vehicle velocity V detected by a velocity sensor
12
, and controls the current to be supplied to the motor
20
based on the computed steering assist command value I. The control unit
30
performs ON/OFF control of the clutch
21
. The clutch
21
is normally in an ON condition (engaged). However, the clutch
21
is in an OFF condition (disengaged) when the control unit
30
determines that the power steering system is at fault, or the power supply from the battery (voltage Vb) is cut-off by the ignition key
11
or the relay
13
.
The control unit
30
consists mainly of a CPU.
FIG. 2
illustrates general functions executed in the CPU by a program. For example, a phase compensator
31
does not represent a phase compensator as an independent hardware component; rather, it represents a phase compensating function executed in the CPU. The function and the operation of the control unit
30
are described below. The torque sensor
10
detects a steering torque T and inputs it to the phase compensator
31
. The inputted steering torque T is phase-compensated for by the phase compensator
31
to enhance the stability of the steering system. Then the phase-compensated steering torque TA is inputted to a steering assist command value computing unit
32
. The vehicle velocity V detected by the velocity sensor
12
is further inputted to the steering assist command value computing unit
32
. The steering assist command value computing unit
32
determines the steering assist command value I, which is equivalent to a control target value of the current to be supplied to the motor
20
, based on the inputted steering torque TA and the velocity V. Then the steering assist command value I is inputted to a subtractor
30
A as well as to a differential compensator
34
of a feed-forward system for increasing a response speed. A deviation (I-i) determined by the subtractor
30
A is inputted to a proportional computing unit
35
and to an integral computing unit
36
. The proportional output and the integral output are both inputted to an adder
30
B. The integral computing unit
36
is used for improving characteristics of a feedback system. The outputs of the differential compensator
34
and the integral computing unit
36
also are inputted to the adder
30
B. As a result, all the inputs to the adder
30
B add up to a current control value E. The current control value E is inputted as a motor drive signal to a motor drive circuit
37
. Finally, a motor current value i of the motor
20
is detected by a motor current detecting circuit
38
, which in turn is fed back through the subtractor
30
A.
Now referring to
FIG. 3
, an example of the structure of the motor drive circuit
37
is shown. The motor drive circuit
37
includes a field-effect transistor (“FET”) gate drive circuit
371
for driving each gate of FET
1
to FET
4
based on the current control value E from the adder
30
B, an H-bridge circuit including FET
1
to FET
4
, and a booster power supply
372
for driving a high side of FET
1
and FET
2
. The FET
1
and FET
2
are switched between an ON condition and an OFF condition by a PWM signal of a duty ratio D
1
, which is determined based on the current control value E, thereby controlling the current Ir actually supplied to the motor
20
. The FET
3
and FET
4
are driven by a PWM signal of a duty ratio D
2
, which is defined by a predetermined linear function formula (given constants a and b, D
2
=a·D
1
+b) in a region where the duty ratio D
1
is of a small value. After the duty ratio D
2
has reached 100%, FET
3
and FET
4
are switched between an ON condition and an OFF condition in accordance with a rotation direction of the motor
20
, which is determined based on the sign of the PWM signal.
There are known electric power steering systems that generate a moderate response in quick steerage of a vehicle. An example of such electric power steering systems is shown in Japanese Patent Publication No.45-41246. The Japanese Patent Publication No.45-41246 describes an apparatus which includes a torsion torque sensor for detecting a torsion torque of a steering shaft when turning a vehicle. In response to the output signal of the torsion torque sensor, the apparatus controls a rotation direction and a rotation torque of an electric motor. However, the above known control apparatus for the electric motor has problems as below. When the output of the control apparatus is set at a high level, convergence of a steering wheel in hand-off steerage of the vehicle deteriorates due to inertia of the control system. In addition, when quickly steering the vehicle around a sharp curve, a driver generally feels more comfortable if there is a moderate response to the steering wheel. Nevertheless, the above known electric power steering system does not include a unit for compensating for an assist steering force (power assist) in accordance with the steering speed. Hence, when making a sharp turn around a curve having a small radius, the driver feels insecure because the steering wheel feels too light.
To solve the above problems, a motor control apparatus, such as disclosed in Japanese Patent No.2568817, is provided wherein brake is applied based on the steering angle of a steering wheel. Specifically, the motor control apparatus for an electric power steering system controls rotation direction and rotation torque of an electric motor that provides a steering mechanism with an assist steering force in accordance with a command signal based on the output signal of a torsion torque sensor for detecting a torsion torque of the steering system. The motor control apparatus includes a detector for detecting a steering angular velocity in the steering system, a steering angle phase-compensating command unit for generating a damping signal, which defines rotation torques in both the steering forward direction and the backward direction, in accordance with the steering angular velocity, and a drive control unit for controlling the ro
Beaulieu Yonel
Cuchlinski Jr. William A.
NSK Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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